FXAK68 PAFC 180130
AFDAFC
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Sat Feb 17 2018
.ANALYSIS AND UPPER LEVELS...
High pressure continues to bring generally stable conditions to
much of the southern Mainland, while upper level cloud cover can
be seen streaming over top of the ridge from a storm force front
moving through the Eastern Bering. Patchy fog and stratus developed
again overnight in Cook Inlet from Kenai northward into Knik Arm
and the Anchorage Bowl, which persisted for much of the morning
today with continued subsidence and weak forcing. From the Eastern
Aleutians northward to the Pribilof Islands, storm force winds
were observed this morning along the front along with widespread
rain which mixed with snow toward the Pribilof Islands. This
front is moving ever so slowly off to the east as quickly as the
mainland ridge allows, but should begin impacting the west coast
by late tonight.
&&
.MODEL DISCUSSION...
The models are in very good agreement with good initialization in
the early term, and are beginning to come to a consensus with the
remnants of the front currently moving through the Eastern Bering
as it tracks into the Gulf/Southcentral late Sunday. Previous runs
had been very inconsistent in either bringing a weaker shortwave
further to the north over top of the flattening ridge over the
Gulf, or digging an upper level low much more aggressively into
the ridge well to the south, which would have diverted much of the
energy/moisture well to the south of Southcentral. This morning's
12z models have all come into agreement in the further north
track, increasing the chances for snow from the Kenai Peninsula
into northward into the Susitna Valley on Sunday evening.
&&
.AVIATION...
PANC...After dissipating this afternoon, the potential for
the redevelopment of LIFR fog/stratus from Northern Cook Inlet
into Knik Arm will be the primary forecast challenge through
Sunday morning. There will be some additional upper level
cloud cover that may inhibit the growth of the fog, but will
also make tracking of it difficult on anything but area webcams,
leading to lower short-term forecast confidence once it begins to
develop late this evening. A weakening front will then bring a
short lived period of MVFR snow Sunday evening with only light
accumulation expected.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
At the onset of the forecast period, the long-wave pattern
features a high amplitude ridge over Kodiak Island that extends
north of the Brooks Range and a low vortex east of Kamchatka. The
upper level ridge will slowly move eastward across southern
Alaska tonight and tomorrow. Patchy fog was detected along Cook
Inlet and Knik Arm early Saturday morning and similar conditions
are anticipated with weak flow in place. With the projected
temperature/dewpoint spreads and the inversions over Southcentral,
patchy fog is possible again Sunday morning. On Sunday, a
vigorous short wave will get kicked out of the vortex over the
Bering and become negatively tilted. This feature will bring
enhanced southwest flow directly up Cook Inlet. The PWs (precipitable
water) spike to nearly 3/4 of an inch near Kodiak, Homer and
portions of Cook Inlet. Between the favorable dynamics and the
moisture surge, bumped up pops this forecast package from Homer to
Palmer, including Anchorage. The timeframe for snowfall in
Anchorage and Palmer is late Sunday and into the early morning
hours of Monday. This is a fast moving shortwave and at this time
significant accumulations are not expected. However, the models
have been bouncing around with the track and the evolution of
this Cook Inlet snow event, so stay tuned for more details in the
event of last minute adjustments.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Tuesday evening)
Increasing winds and precipitation is the main item of interest
for the next 24 to 36 hours, as a weak cold front moves ashore.
Ahead of this feature, southeast down-slope winds have warmed the
temperatures higher than model guidance indicated earlier this
morning. Thus, the morning update for freezing rain across the
western zones has been removed with this afternoon's package.
With the warmed air, delayed the onset of blowing snow as well.
Otherwise, look for widespread rain to develop with increasing
winds as the front approaches.
Strong wind gusts are already occurring along the coast, and this
will continue until frontal passage with some gusts in the 45 to
60 mph range expected near the coast. The models have also trended
faster with the colder air moving in behind the front. Most
locations should see 2 to 5 inches of snowfall, with the mountains
northwest of Dillingham seeing perhaps a touch more. Conditions
look to dry out for Sunday before another front brings more wind
and precipitation back to the region by Monday.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Tuesday evening)
An active pattern will continue through early next week, with
multiple lows/fronts affecting the region. Widespread rain and
wind gusts to 60 mph will continue through Sunday morning before
the system pushes east of the area. A shortwave ridge will quickly
advance from west to east for Sunday evening into Monday, bringing
lighter winds and some clearing. This will be short-lived though,
as a strengthening low pressure system moves from the north
Pacific into the western Bering, sending another front eastward.
High end gales will be fairly widespread with some locations
seeing sustained storm force conditions. Given the repetitive
pattern and good fetch length, seas will build into the 30 to 35
feet range across the western Bering/coastal waters of the chain.
Widespread moderate to locally heavy precipitation will accompany
the front as well, with strong wind gusts likely over land as the
front progresses eastward.
&&
.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast beginning Monday night starts with dry
conditions across Southcentral with a surface ridge across the
eastern interior. To the west, an organized low tracking to the
northern Bering moves a front from the eastern waters to the
Southwest, spreading light snow inland before diminishing just
west of the Alaska Range early Tuesday. A ridge amplifies across
the southern mainland late Tuesday into Wednesday ahead of the
next approaching system.
Another storm system tracks quickly to the southwest Bering early
Tuesday on a similar track toward the Bering Strait. This system
will be "the one to watch" this week as a significant short-wave
travels to the southern mainland late Wednesday into Thursday,
bringing increased chances for precipitation as it propagates to
the Alcan border. The pattern remains active through the next work
week as a third storm system enters the western Bering Wednesday
night pushing the upper long wave eastward.
Models are in general agreement with the active forecast pattern
through the end of next week, however they differ with the
amplitude of the upper wave pattern and in the progression of the
storm systems. The updates for the current package maintain
consistency, but higher uncertainty remains through the extended
as models typically struggle with these active patterns.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 173 175 176.
Gale Warning 150 155 160 165 170 171 172 174 177 178 179 180 181
185.
&&
$$
SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...KH